/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.jetbrains.builtInWebServer.ssi;

/**
 * Parses an expression string to return the individual tokens. This is
 * patterned similar to the StreamTokenizer in the JDK but customized for SSI
 * conditional expression parsing.
 *
 * @author Paul Speed
 */
public class ExpressionTokenizer {
  public static final int TOKEN_STRING = 0;
  public static final int TOKEN_AND = 1;
  public static final int TOKEN_OR = 2;
  public static final int TOKEN_NOT = 3;
  public static final int TOKEN_EQ = 4;
  public static final int TOKEN_NOT_EQ = 5;
  public static final int TOKEN_RBRACE = 6;
  public static final int TOKEN_LBRACE = 7;
  public static final int TOKEN_GE = 8;
  public static final int TOKEN_LE = 9;
  public static final int TOKEN_GT = 10;
  public static final int TOKEN_LT = 11;
  public static final int TOKEN_END = 12;
  private final char[] expr;
  private String tokenVal = null;
  private int index;
  private final int length;

  /**
   * Creates a new parser for the specified expression.
   */
  public ExpressionTokenizer(String expr) {
    this.expr = expr.trim().toCharArray();
    this.length = this.expr.length;
  }

  /**
   * Returns true if there are more tokens.
   */
  public boolean hasMoreTokens() {
    return index < length;
  }

  /**
   * Returns the current index for error reporting purposes.
   */
  public int getIndex() {
    return index;
  }

  protected boolean isMetaChar(char c) {
    return Character.isWhitespace(c) || c == '(' || c == ')' || c == '!'
           || c == '<' || c == '>' || c == '|' || c == '&' || c == '=';
  }


  /**
   * Returns the next token type and initializes any state variables
   * accordingly.
   */
  public int nextToken() {
    // Skip any leading white space
    while (index < length && Character.isWhitespace(expr[index])) {
      index++;
    }
    // Clear the current token val
    tokenVal = null;
    if (index == length) return TOKEN_END; // End of string
    int start = index;
    char currentChar = expr[index];
    char nextChar = (char)0;
    index++;
    if (index < length) nextChar = expr[index];
    // Check for a known token start
    switch (currentChar) {
      case '(' -> {
        return TOKEN_LBRACE;
      }
      case ')' -> {
        return TOKEN_RBRACE;
      }
      case '=' -> {
        return TOKEN_EQ;
      }
      case '!' -> {
        if (nextChar == '=') {
          index++;
          return TOKEN_NOT_EQ;
        }
        return TOKEN_NOT;
      }
      case '|' -> {
        if (nextChar == '|') {
          index++;
          return TOKEN_OR;
        }
      }
      case '&' -> {
        if (nextChar == '&') {
          index++;
          return TOKEN_AND;
        }
      }
      case '>' -> {
        if (nextChar == '=') {
          index++;
          return TOKEN_GE; // Greater than or equal
        }
        return TOKEN_GT; // Greater than
      }
      case '<' -> {
        if (nextChar == '=') {
          index++;
          return TOKEN_LE; // Less than or equal
        }
        return TOKEN_LT; // Less than
      }
      default -> { }
      // Otherwise it's a string
    }
    int end = index;
    if (currentChar == '"' || currentChar == '\'') {
      // It's a quoted string and the end is the next unescaped quote
      char endChar = currentChar;
      boolean escaped = false;
      start++;
      for (; index < length; index++) {
        if (expr[index] == '\\' && !escaped) {
          escaped = true;
          continue;
        }
        if (expr[index] == endChar && !escaped) break;
        escaped = false;
      }
      end = index;
      index++; // Skip the end quote
    }
    else if (currentChar == '/') {
      // It's a regular expression and the end is the next unescaped /
      char endChar = currentChar;
      boolean escaped = false;
      for (; index < length; index++) {
        if (expr[index] == '\\' && !escaped) {
          escaped = true;
          continue;
        }
        if (expr[index] == endChar && !escaped) break;
        escaped = false;
      }
      end = ++index;
    }
    else {
      // End is the next whitespace character
      for (; index < length; index++) {
        if (isMetaChar(expr[index])) break;
      }
      end = index;
    }
    // Extract the string from the array
    this.tokenVal = new String(expr, start, end - start);
    return TOKEN_STRING;
  }

  /**
   * Returns the String value of the token if it was type TOKEN_STRING.
   * Otherwise null is returned.
   */
  public String getTokenValue() {
    return tokenVal;
  }
}